1. Field of the Invention
This invention relates generally to treatment of waste liquids; and more particularly to a new and improved method and apparatus for treating waste water and other liquids utilizing dissolved air flotation and related technologies.
2. Description of the Prior Art
Conventional methods of treating waste water and similar liquids is usually carried out in several stages. Easily settled solids are separated from a liquid medium by sedimentation, and suspended solids and emulsified matter are removed by unassisted flotation or dissolved air flotation (DAF). Remaining dissolved matters may be converted chemically to precipitates, or biologically to microorganisms, for separation from the liquid medium. Sometimes dissolved matters are removed by mass transfer operations, such as adsorption, desorption, stripping, extraction, crystallization, membrane separation, etc. The solids separated by these treatments is usually dewatered before incineration or by other disposal means.
The equipment employed in each stage of treatment requires considerable capital investment and plant space, especially if the treatment involves aerobic or anerobic biotreatments of dissolved organics or oxidative digestion of microorganisms. Such treatments are not possible where there is limited capital and space.
Most of these systems are also unacceptable economically and environmentally. They are relatively inefficient because of their low conversion rates, due mainly to turbulence and back-mixing in the liquid flowpath, especially at high flow rates. The pervasiveness of the turbulence and back-mixing is discussed in more detail in U.S. Pat. No. 5,382,358 to George C. Yeh entitled "Apparatus for Dissolved Air Flotation and Similar Gas-Liquid Contacting Operations."
Conventional DAF systems typically place baffles inside a flotation tank in front of the incoming liquid medium to direct floc-bubble agglomerates formed therein toward the surface. In an upright cylindrical tank configuration, the baffle is generally placed concentrically and upright in the tank. In a rectangular tank configuration, such as disclosed in U.S. Pat. No. 3,175,687 to W. H. Jones, the baffle is flat and placed generally upright. The turbulence, recirculation and back-mixing produced by the baffles more than offsets the beneficial effect of directing the agglomerates to the surface, especially at high flow rates or sudden surges in flow. Consequently, the liquid loading capacity is limited, and clarification efficiency degraded because the incoming liquid continues to flow freely under turbulent conditions after passing the baffles. While such baffles in both cylindrical and rectangular flotation tanks help to direct floc-bubble agglomerates toward the surface of the liquid medium, their total effect is detrimental to DAF systems.
The effects of turbulence and back-mixing can be reduced by increasing the length of the liquid flowpath over which flotation occurs, such as by a long rectangular tank with liquid inlet and outlet ports at either end, but space, especially if indoors, is insufficient to accommodate such a long tank. Another tank designed for smaller areas utilizes a short rectangular tank divided lengthwise into a connecting series of labyrinthine-like compartments through which the liquid flows in a winding path. However, the turbulence created at the junctions of these compartments, and the restrictions in the number of compartments possible, limit the beneficial effects.
U.S. Pat. No. 5,382,358 supra discloses a unique design in which an upright cylindrical flotation tank includes at least one partition curved forming a long involute or evolute flow channel. A mixture of untreated liquid and recycled liquid saturated with dissolved air under pressure is continuously introduced at a controlled rate at one end of the channel and flows through the channel under near plug-flow conditions. Micro-bubbles of air released from the mixture rise through the liquid mixture providing a buoyant force which carries any contacted particles of suspended matter to the surface. A sludge is formed and the top layer of which is continuously skimmed off at a rate regulated to maintain a predetermined residence time and solid content. Centrifugal and gravitational forces in the flowpath cause large or heavy solids to separate and settle in the bottom of the tank where they may be drawn off. Commercial units constructed according to the patent have achieved unusually high clarification efficiencies, viz. at or near 100% by weight, and solid contents in the sludge between 50% and 85% by weight depending on the sludge residence time on the surface.
The involute or evolute flotation tank configuration is also applicable to gas-liquid contacting processes, mass transfer processes between gases and liquids, chemical and biological gas-liquid reactions, and for two or more of these processes carried out simultaneously. No other DAF systems are known to be capable of carrying out multiple processes simultaneously.
While the involute or evolute configuration tank has proven a significant improvement in the state of the art, there remain new DAF facilities where rectangular flotation tanks are more suitable because of lower initial costs, anticipated loadings, nature of the liquid medium, etc. There are also many existing DAF facilities having rectangular flotation tanks in need of upgrading for more efficient operations.